Thrombocytopenia is, after anemia, the most common hematological problem among sick neonates, affecting 20-35% of NICU patients. Platelet transfusions (PLTXs) are frequently given to affected neonates, in an effort to reduce the incidence or severity of hemorrhages. Recently, the FDA approved two novel thrombopoietin (TPO) mimetics for the treatment of adults with immune thrombocytopenic purpura. Based on the duration and severity of thrombocytopenia, up to 10% of thrombocytopenic neonates could also benefit from these agents. However, their use in this population has been hampered by a lack of information about differences between neonates and adults with respect to the pharmacokinetics, pharmacodynamics, and toxicities of these agents, and by our inability to predict the duration of neonatal thrombocytopenia. Our long term goal is to improve the treatment of thrombocytopenic neonates through the evidence-based use of PLTXs and TPO mimetics. Toward this goal, we aim to complete the preclinical in vitro and in vivo studies necessary to guide the use of TPO mimetics in newborn infants. Our central hypothesis is that parameters generated through an advanced mathematical model of neonatal thrombopoiesis will enable us to identify neonates who will benefit from TPO mimetics, but that the cellular, molecular, and pharmacodynamic responses of neonates will be different from those of adults. To test this hypothesis, we have formulated the following Specific Aims: 1) To characterize the effects of TPO and two TPO mimetics, romiplostim and eltrombopag, on fetal, neonatal, and adult human megakaryocytes (MKs). 2) To establish the rates of platelet production at baseline and following the administration of romiplostim in newborn mice with thrombocytopenia; 3) To determine whether romiplostim and/or eltrombopag cross the blood-brain barrier; and 4) To develop clinically useful parameters to predict the duration of thrombocytopenia in neonates. These aims will be accomplished using cellular and molecular studies on primary human MKs (S.A. 1), novel murine models of neonatal thrombocytopenia (S.A. 2), serial cerebrospinal fluid studies in newborn lambs (S.A. 3), and mathematical models of neonatal thrombopoiesis (S.A. 4). The first 3 aims will lead to a better understanding of the neonatal responses to TPO mimetics, and of the potential of these agents to cross the blood-brain barrier (this will be critical given the pro-apoptotic effects of TPO on neurons). The studies in S.A. 4 will develop parameters that are predictive of prolonged thrombocytopenia in neonates. Overall, Project 2 is highly synergistic with the other PPG projects in that it applies concepts, methods and mathematical models developed by PPG investigators to evaluate neonatal erythropoiesis and EPO biology to the study of neonatal thrombopoiesis and TPO mimetics, thus allowing the field of neonatal thrombocytopenia to advance exponentially.